The present technique relates generally to the field of power supply, such as that to motor control centers (MCCs). Specifically, the invention relates to techniques for connecting incoming power supply to certain types of electrical machinery, such as MCC's and components, for protecting such connections, and for containing and extinguishing arcing within such systems when faults do occur.
Systems that distribute electrical power for residential, commercial, and industrial uses can be complex and widely divergent in design and operation. Electrical power generated at a power plant may be processed and distributed via substations, transformers, power lines, and so forth, prior to receipt by the end user. The user may receive the power over a wide range of voltages, depending on availability, intended use, and other factors. In large commercial and industrial operations, the power may be supplied as three phase ac power (e.g., 208 to 690 volt ac, and higher) from a main power line to a power management system. Power distribution and control equipment then conditions the power and applied it to loads, such as electric motors and other equipment. In one exemplary approach, collective assemblies of protective devices, control devices, switchgear, controllers, and so forth are located in enclosures, sometimes referred to as “motor control centers” or “MCCs”. Though the present technique is discussed in the context of MCCs, the technique may apply to power management systems in general, such as switchboards, switchgear, panelboards, pull boxes, junction boxes, cabinets, other electrical enclosures, and so forth.
The MCC may manage both application of electrical power, as well as data communication, to the loads, such loads typically including various machines or motors. Within the MCC may be disposed a variety of components or devices used in the operation and control of the loads. Exemplary devices contained within the MCC are motor starters, overload relays, circuit breakers, and solid-state motor control devices, such as variable frequency drives, programmable logic controllers, and so forth. The MCC may also include relay panels, panel boards, feeder-tap elements, and the like. Some or all of the devices may be affixed within various “units” (or “buckets”) within the MCC. The MCC typically includes a steel enclosure built as a floor mounted assembly of one or more vertical sections containing the units or buckets. An MCC vertical section may stand alone as a complete MCC, or several vertical sections may be bolted and bused together. Exemplary vertical sections common in the art are 20 inches wide by 90 inches high.
The MCC normally interfaces with (and contains) power buses and wiring that supply power to the units and components. For example, the MCC may house a horizontal common power bus that branches to vertical power buses at each MCC vertical section. The vertical power buses then extend the common power supply to the individual units or buckets. To protect the power buses from physical damage, both the horizontal and vertical buses may be housed in enclosures, held in place by bus bracing or brackets, bolted to molded supports, encased in molded supports, and so forth. Other large power distribution equipment and enclosures typically follow a somewhat similar construction, with bus bars routing power to locations of equipment within the enclosures.
To electrically couple the MCC units or buckets to the vertical bus, and to simplify installation and removal, the units may be provided with self-aligning electrical connectors or metal stabs on the back of each unit. To make the power connection, the stabs, which may comprise spring-supported clamp devices, engage metal bars disposed on the vertical bus. For three phase power, three stabs per unit may accommodate three bus bars for the incoming power to give the phase terminals or terminations at the unit. An optional ground bus may also be used. Within the unit, three stab wires or power lead wires may route power from the stabs to a disconnecting device or component, typically through protective devices such as fuses and circuit breaker. It should be noted that though three phase ac power is discussed, the MCCs may also manage single phase ac power, as well as dc power (e.g., 24 volt dc power for sensors, actuators, and data communication). Moreover, the individual units or buckets may connect directly to the horizontal common bus by suitable wiring and connections.
A problem in the operation of MCCs and other power management systems, such as switchboards and panelboards, is the occurrence of arcing (also called an arc, arc fault, arcing fault, arc flash, arcing flash, etc.) which may be thought of as an electrical conduction or short circuit through gas or air. Initiation of an arc fault may be caused by a momentary or loose connection, build-up of foreign matter such as dust or dirt mixed with moisture, insulation failure, or a short-circuit (e.g., a foreign object establishing an unwanted connection between phases or from a phase to ground) which causes the arc to be drawn, and so forth. Once initiated, arcing faults may proceed in a substantially continuous manner. On the other hand, arcing faults may be intermittent failures between phases or phase-to-ground, and may be discontinuous currents that alternately strike, extinguish, and strike again.
In either case, the result is an intense thermal event (e.g., temperatures up to 35,000° F.) causing melting and vaporization of metals. An arcing fault is an extremely rapid chain of events releasing tremendous energy in a fraction of a second, and is known for quick propagation. Once the arcing begins, heat is generated and ionized gases are produced that provide a medium by which the arcing fault can propagate. An arc may travel along one stab wire and jump to other stab wires, melting and/or vaporizing the stab wires. As a result, more ionized gas and arcing may be created, engulfing all three phases and possibly reaching the power buses. A phase-to-ground or phase-to-phase arcing fault can quickly escalate into a three-phase arcing fault due to the extensive cloud of conductive metal vapor which can surround the power leads and terminals. If not contained, the arc may propagate throughout the entire MCC, especially if the arc reaches the power buses. Arcing faults can cause damage to equipment and facilities, and drive up costs due to lost production.
It has been well documented that incident energy of an arcing fault is directly proportional to the time the fault persists. As the arcing fault flows for 6, 12, or 30 cycles or more, for example, the incident energy and force of the arc fault increases dramatically. Thus, circuit breakers, for example, on the line side operating with typical time delays (e.g., greater than 6 cycles) may be problematic with arcing faults. In general, it is desirable that the arcing fault be extinguished in a short time, such as within 6 cycles, and in certain applications, in less than 2 cycles. Testing has shown that if the arc (e.g., for 65,000 amps available current at 480 volts) does not extinguish quickly (e.g., in less than 0.1 seconds or six cycles), it can cause extensive damage. Moreover, although the amount of energy released in an arc flash may be greater for higher voltage installations, such as those found in petrochemical and other industrial plants, the sheer volume of lower voltage equipment in commercial and industrial facilities means that such installations account for a great number of arc flash incidents. Thus, there has been interest in arc flash protection for medium and low voltage MCCs, in addition to interest for protection of high voltage systems. Finally, as known by those skilled in the art, there are several industry and regulatory standards around the world that govern arc flash prevention.
Arc characteristics and incident energy levels have many variables, such as system voltage, arc current, arc duration, arc electrode spacing, and so forth. In recent years, significant progress has been made in understanding arcing faults. For example, analytical tools have been developed to better assess arcing faults. As a result, it has been found that current-limiting devices, low impedance circuit components such as low impedance transformers, reduce the occurrence of arcing faults and/or the arc energy. However, such advances have proved deficient in mitigating arcing fault incidents.
There is a need, therefore, for improved stab housing and enclosure designs that reduce the potential of arcing faults going phase-to-phase and reaching the power buses. Similarly, there is a need for a technique that efficiently and quickly extinguishes arcing faults to reduce damage to the MCC and other power management systems.